Your question is rather broad, so I would suggest you have a look at our U.S.A guide. Have a look at the states featured, which can hopefully provide some advice for which place best suits your needs.
I would say Atlanta is considered a "dirty city" over here, so I'd avoid that.
DC is the capital, is very walkable, but also can be dirty and has high crime rates.
New York is a very cool, safe, clean, and walkable city. It has a ton to do, and is very busy, but is also very cold during the winter, and the likeablity of this city is heavely dependent on personal opinion.